Researchers map mixing in the Southern Quark

At present, a unique research expedition is taking place outside Grisslehamn, Sweden with Stockholm University’s research vessel Electra. Eight researchers from Stockholm University, the German Leibniz Institute for Baltic Sea Research (IOW) and the Swedish Meteorological and Hydrological Institute (SMHI) studies highly energetic flow over the steep seafloor topography in the Southern Quark, between Åland and Sweden.

- Our goal is to map the vertical mixing of water masses caused by turbulence close to the steep underwater mountain ridges. The overarching research question for this project is where and how vertical mixing is happening says Christian Stranne, Assistant Professor in marine geophysics at the Department of Geological Sciences, Stockholm University.

Vertical mixing of water is important for the transport of salt and nutrients from the seafloor towards the surface. The near-bottom water of the deeper basins in the Baltic Sea are often oxygen depleted and this leads to a release of large amounts of nutrients from the sediments. By means of complex and not fully understood transport mechanisms, these nutrients reach the surface water in winter where they reinforce the already problematic eutrophication of the Baltic Sea.

- Vertical transport of water and nutrients from deep waters in the Baltic Sea can be estimated through salinity and volume budget calculations. These estimates are about ten times higher than those from direct measurements of vertical mixing. The identification and quantification of this “missing mixing” is a major open question in Baltic Sea research, and one that we are focusing on in the present project, Stranne explains.

Highest turbulence levels recorded

There is good reason to believe that this particular area in the Southern Quark is a hotspot for turbulent mixing. In 2019, the research team carried out a pilot project in the area with overwhelming results.

- The topography is wild in this area and we measured some of the highest levels of turbulence ever observed in the Baltic Sea. There is a lot going on under the sea surface in this area, Stranne says.

This knowledge is important. Present ocean models generally fail to predict where and when vertical mixing in the ocean occurs, partly due to the limited model resolutions in relation to the patchy and intermittent nature of turbulence, but also, and perhaps more importantly, due to knowledge gaps in our understanding of these processes.

- Today’s models can handle smooth topography but it is very computationally demanding to resolve flow over rough topography. That’s why the acoustic observations are a fantastic complement. It is amazing to go out at sea and see what is actually happening in the water column, says Julia Muchowski, PhD student in marine geophysics in the Department of Geological Sciences, Stockholm University.

Equipment-heavy research expedition

The current expedition carries a heavy load of equipment. For one, the packing list includes 400 kg of railway track that will serve as bottom weights for three instrument moorings that will be deployed on the seafloor during the time of the expedition. To get a detailed picture of the energetic overflow and mixing in the area, the team will combine established, labor intensive measurements of turbulence (taken directly in the water) with acoustic observations.

- With the acoustics, we can add important information to the direct measurements in the water column. What is fantastic with Electra is that she is equipped with state-of-the-art acoustics such as a Simrad EK80 wideband echo sounder. Since she is a smaller research vessel, Electra is much quieter than other ships which makes our data very clean, Christian Stranne concludes, and exemplifies with this video:

About the project:

The research project ”Studies of ocean mixing in relation to steep bottom topography with novel acoustic methods” is a collaboration between Stockholm University, Leibniz Institute for Baltic Sea Research (IOW) and SMHI. The expedition with R/V Electra occurs between February 25 and March 6. Follow the route at Marine Traffic.